In the documentary “the secrets of sleep” there is a segment about a guy who was in a coma for years and had dreams. Interestingly, there was the regulat occurrance of a state when everything was either black or white, sort of like the void. When I saw it I thought that it could be cycling through some different stages.
It’s not hard to imagine a comatose state where one has some consciousness, but no sensory input. Sensory deprevation causes hallucinations.
oh and. about brain surgery. I’m sure they have the patients awake for it. with only local anasthetic. because the brain is so sensitive, they test it with an electrical thingy, and can map out the patients individual structure. It they didn’t they might destroy functions needlessly. At least that’s what they said in another documentary, backed up by footage.
I can answer this with some confidence because that is when my dreaming became so odd, after my coma!! However, I was in a medically induced coma because my body was dying and needed to be completely rested. They did not expect me to wake up. I can tell you the brain is most CERTAINLY active during coma but I am guessing experiences will differ depending on whether coma was induced or as result of accident/injury to brain and also due to the fact it is a very very personal thing. I am sure no two people ever have the same experience. 
An induced coma is brought about by medication and therefore the experiences may not be very reliable, also, as I already mentioned, every person differs wildly due to personality, lifestyle, upbringing, beliefs etc etc. I only know one other person who has been in a coma and his, although also induced, was a very different experience to mine regarding what he saw heard and felt BUT he did (like me) have very vivid real ‘memories’ of everything he experienced. The problem for me is that my mind must have filled in the time I would usually have been awake and left memory imprints. I woke up thinking I had a son, really believing it was real. I also had vivid ‘memories’ of places I had been, odd surreal places, fictional/spiritual characters entwined with people from my real life such as my daughter. Most of my ‘memories’ have faded but not gone and I still, nearly 4 years later, have difficulty accepting that some of what I saw really was not real and did not happen. 
Its complicated by the fact that some of the procedures and examinations filtered through, I remember things happening to me, felt & saw things I later discovered coincided with medical procedures I had whilst in the coma state, my mind turned it into something I would understand I guess but dont trust that people in comas feel nothing it simply is not the case. One horrible example is that I was assaulted, I really believed it had happened but when I described it to my Mother she explained some of the neccesary procedures they had to do and it was clear that it was those exams and procedures I actually felt, my mind created the reality for me, the experience was real the circumstance was not. 
I dont know if what I experienced can be called dreams or hallucinations or something else. I do know that I was very frightened by some of it, it was real and still confuses me now trying to seperate the reality from the dreaming. I lived those months in that coma in some other existance, I have memories of those months and the things I did yet I was ‘asleep’ I dont know what any of it shows or proves but my LD experiences are much more frequent and vivid since that time. I dont think my mind will ever be the same, I almost feel like I saw things that are absolutely real, we just dont know they are real until we leave this life I guess. I am certain there is another reality, one that may explain OOBE as I had those in the coma also. There is a connection between LD and OOBE but I just cant define it. 
Sorry to ramble on but I never had the chance to express any of this before & was specifically looking for information regarding comas and our ability to dream, what kinds of dreaming and how that may affect someone in their real life afterwards. I saw a lot of scary negative stuff, the person I mentioned earlier, a man who was also in an induced coma, 
he saw highly positive spiritual things that actually changed his whole beliefs when he woke. Why one should see good and one bad I dont know, its not related to our lives as neither of us have done anything particularly spectacular either good or bad. The thing is he insists, same as me, that some of it was very very real, a memory not a dream. He sees it vividly if asked to recall it as though yesterday. Both of us now believe strongly that there is something other than this reality beyond what we see. Both of us have had our sleep changed seemingly permanently and have regular LD episodes and dream experiences that make us question our current reality. 
After all if you had lived a life for three months, a real sensory emotional life, had exact experiences as in your real life, spent time with people, eaten slept woke, carried on your day however odd it may have got at times only to wake up one morning and be told you had actually been asleep that whole three months, wouldnt you sometimes question if you are indeed awake right now or still sleeping, waiting to wake up back in the real world whatever that may be!!! Its a very very odd sensation indeed! 